This invention relates to foul release coatings and articles coated therewith. More particularly, it relates to sprayable foul release coatings which may be employed without undue environmental harm.
As poetically stated in U.S. Pat. No. 4,861,670, "Marine fouling due to pernicious and pestiferous sessile organisms is a problem which reaches from ancient times to the present." In more simple terms, a perennial major aggravation to shippers and users of marine equipment in contact with water is the tendency of such equipment to become encrusted with various varieties of wildlife, as illustrated by barnacles and zebra mussels.
Said patent goes on to describe in considerable detail the types of treatments that have been employed, starting as early as 1854, to minimize marine fouling. Treatment materials have included compounds of such metals as copper, tin, arsenic, mercury, zinc, lead, antimony, silver and iron, as well as toxic organic materials such as strychnine and atropine. With increasing interest in the state of the environment, the use of such materials has been strongly discouraged.
More recently, polyorganosiloxanes (hereinafter sometimes designated "silicones" for brevity) have been found useful as anti-fouling coatings. They include condensation cured silicones, which, however, are sometimes considered undesirable since a frequent constituent thereof is a catalyst containing tin, and tin in various states is believed to have undesirable effects on aquatic life. Interest has turned to some extent, therefore, to the use of addition curable silicones, which are typically cured by the action of platinum-containing catalysts in very small proportions.
The aforementioned U.S. Pat. No. 4,861,670 discloses a suitable addition curable silicone coating material which may be employed for foul release in marine environments. It comprises a vinyl chain-stopped polysiloxane in combination with a non-reinforcing filler, a platinum catalyst, an organohydrogen polysiloxane and, as a reinforcing material, an organopolysiloxane copolymer comprising units generally described by the formulas R.sub.3 SiO-- and Si(O--).sub.4. Such units are conventionally and hereinafter sometimes designated M and Q units, as abbreviations for "mono" and "quater" representing the number of Si--O moieties therein, and such a copolymer is correspondingly designated an MQ resin. By the same rationale, R.sub.2 Si(O--).sub.2 and RSi(O--).sub.3 units are respectively designated D and T for "di" and "tri".
The MQ organopolysiloxane employed in the patented compositions functions as a reinforcing filler material. At the same time, however, its rheology is such that the coating composition in which it is incorporated is thixotropic and thus incapable as prepared of application by spraying. As disclosed in said patent, such compositions can only be conveniently applied neat by such time-honored but tedious methods as brushing or roller coating. The only potential way to make such materials sprayable is to dilute them with solvents, typically volatile organic compounds such as hydrocarbons which present their own environmental hazards. In any event, many localities have enacted legislation limiting the proportions of volatile organic compounds in such compositions to values in the parts-per-million range, far below those that would be necessary for them to serve as solvents.
Thus, the development of sprayable, environmentally harmless addition curable silicone foul release coating compositions remains a major concern.